The invention is a selective calling circuit that is more particularly described as an asynchronous circuit using consecutive repetitions of selected digital code words.
Selective calling systems are useful in many applications, one of which is for monitoring the status of equipment, such as protection switches, located at unmanned remote stations along a transmission line and for controlling operation of that equipment. Each remote station includes a selective calling decoder for determining whether or not a central control station is calling the particular remote station. In response to receipt of a call, the remote station responds by either transmitting information regarding the status of some equipment or condition at the remote station, or by applying a control signal to equipment located at the remote station.
For the remote station, a wide variety of selective calling arrangements have been described in the prior art. They include multiple frequency selection schemes, synchronous digital code schemes, and repetitive asynchronous digital schemes. Designers having turned from multiple frequency schemes to digital code schemes because the multiple frequency schemes require complicated base station equipment when the system is arranged to call a large number of remote station codes. The digital code scheme arrangements disclosed in the prior art generally require complicated timing arrangements for both the central control station and the remote station.
The prior art arrangements generally are complicated and have the disadvantage of high cost. Some selective calling arrangements, for monitor and control systems requiring only a few calling codes, may be prohibitively expensive when complicated arrangements of the prior art are used. Thus, there is a need for an uncomplicated selective calling system that will selectively call a small number of remote station codes at a low cost.
Therefore it is an object to provide a simple selective station calling arrangement using a simple coding scheme.
It is another object to provide a low cost selective station calling arrangement.
It is a further object to provide an asynchronous selective calling arrangement.
It is still another object to provide a selective calling arrangement using a code without framing.